One known way to realize a low cost, yet high performance, disk drive is to provide a head positioner servo control loop which operates upon positional samples periodically read by the head from servo sectors embedded within concentric data tracks formed on an adjacent rotating data storage surface over which the head is presently passing. In this example, cost savings are realized by avoidance of a dedicated servo data surface, servo transducer and dedicated servo data detection channel in the disk drive.
During track seeking operations, position information is sampled from the sectors as the head moves from a departure track to a destination track. During track following operations, successive servo samples are taken from the servo sectors of a single data track being followed by the head under servo loop control. The sampled positional information is then compared by the digital servo control system with predicted positional location, and any deviation or error results in a correction current applied to a voice coil actuator and resultant corrective movement of the head-carrying actuator structure.
Increasingly, sampled servo head positioners are implemented with digital signal processing techniques. One known technique is to employ state estimation techniques for estimating parameters of the head positioning process. One example of this approach is provided in U.S. Pat. No. 4,679,103 to Workman entitled: "Digital Servo Control System for a Data Recording Disk File". The Workman patent discloses a digital servo control system implemented with a microprocessor. The system receives a digital head position error signal and a digital signal corresponding to the head actuator current. The system responds to these inputs by generating and putting out a digital control signal which is converted to analog and amplified before being applied to drive the actuator. The control signal is calculated from estimated values of absolute head position, head velocity and an equivalent actuator current required to compensate for constant or very low frequency actuator bias forces. The estimated head position, velocity and acceleration for each digital sample are functions of the predicted head position, velocity and acceleration, the measured head position, and the measured actuator coil current. These predicted values are updated with each sample and are derived from functions based upon a physical model of the voice coil actuator structure, taking into account its rotary inertia, friction, etc.
In the implementation of this digital approach, a considerable number of calculational steps must be carried out in a processing interval related to the servo sample interval. For example, the method described by the Workman '103 patent involves the following digital processing steps for each servo error signal sample:
1. generating from the servo information a sampled head position error signal representative of the position of the head relative to a centerline of the nearest track;
2. estimating the absolute head position relative to a reference, and estimating the head's radial velocity;
3. estimating the equivalent actuator input signal required to compensate for bias forces acting upon the actuator structure;
4. measuring the actuator input current;
5. computing a commanded head velocity as a function of the distance from the estimated absolute head position to the destination head position at a target track;
6. estimating the head radial acceleration as a function of the measured actuator current and the estimated bias force;
7. generating a control signal as a function of the head velocity error, the estimated head acceleration and the control signal generated for the prior servo error signal sample; and applying the control signal via a digital to analog converter to an actuator power amplifier which powers the actuator coil;
8. storing the estimated absolute head position, the estimated head velocity, the estimated bias force equivalent actuator input current, the measured actuator coil current, and the control sample;
9. predicting the absolute head position and the head velocity as functions of prior control signals, the prior estimated absolute head position, the prior estimated head velocity, the prior estimated bias force equivalent actuator input current, and constraints representative of a physical model of the actuator structure, the predicted absolute head position and velocity for each servo error signal sample being used during the step of estimating the absolute head position and head velocity for the subsequent error signal sample; and
10. predicting the bias force equivalent actuator input signal as a function of the prior estimated bias force actuator input current, the predicted bias force equivalent actuator input current for each servo error signal sample being used during the step of estimating the bias force equivalent actuator input current for the subsequent servo error signal sample.
This process also typically includes digital notch filtering steps for filtering out any resonances in the actuator structure in order to stabilize the servo control loop. As is suggested by this extensive list of required steps, a considerable amount of computation and related data manipulation is required to implement the Workman method. Further elaboration and practical design examples of digital control theory applicable to servo control of head positioning in disk files are found in Franklin, Powell and Workman, Digital Control of Dynamic Systems, 2d Ed., Addison-Wesley Publishing Company, New York, N.Y. .RTM.1990, especially at pp 703-775: "Case Design: Disk-Drive Servo".
One performance improvement in the state estimator proposed in the Workman '103 patent (as it may be applied to embedded sector servo disk drives) is suggested in a paper by Wen-Wei Chiang, entitled: "Multirate State-Space digital controller for Sector Servo Systems", IEEE Proc. 29th Conf on Decision and Control. Honolulu, Hi. December 1990, pp. 1902-1907. This paper projects improvements in servo performance when the digital state estimator is made to operate at a rate which is an integer multiple (e.g. 2, 3, or 4 times) of the sector servo sample rate, so that the state feedback control of the actuator current may be computed and updated more frequently. One example of a practical disk drive design employing a multirate digital servo loop is provided in the present inventor's commonly assigned, copending U.S. patent application Ser. No. 07/954,557, filed on Sept. 30, 1992, and entitled: "Disk Drive Having On-Board Digital Sampling Analyzer"; the disclosure of this application is incorporated herein by reference.
From the foregoing, it is immediately apparent that the digital signal processor must inherently have sufficient processing power to carry out all of the process steps outlined above in multiple iterations within a single servo sampling interval. From a practical point of view, a multirate servo control process requires a high clock speed digital processor dedicated solely to making the computations and related data manipulations required to provide more frequent control signal updates. The significant signal processing capability required to realize a multirate servo loop may not be practical in lower cost disk drive designs which e.g. employ a single, multitasked digital microprocessor which divides its time between servo control functions and other functions such as supervision of data transfers and host-disk communications.
As suggested above, every servo actuator structure manifests a mechanical resonance at some frequency or frequencies. These vibrational modes, unless accounted for in the servo control design, can result in servo control loop instabilities. One known way to reduce the adverse effects of actuator resonance is to provide a notch filter in the servo control loop centered at the resonance frequency. A recent example of an analog notch filter providing some adjustability within a disk drive is found in U.S. Pat. No. 4,936,806.
It is known that in a discrete time domain (sampling) system, the system control transfer function manifests a minimum value at the sampling frequency. This characteristic has been proposed for use as a notch filter function in a head position servo control loop in order to filter out head actuator resonance, see, e.g., Bauck U.S. Pat. No. 4,398,228 entitled "Method of Avoiding Resonance in Servo Controlled Apparatus". One apparent drawback of this approach is that the sampling frequency must be chosen strictly in relation to the mechanical resonance of the actuator. Such a selection may not be optimum, given other design constraints. For example, when the actuator structure is varied, as by changing the number of disks/heads in the drive, a different sampling frequency and different overall servo control loop design would be required, even within the same product family design. Another inherent drawback is the inability of the Bauck servo loop to sense and respond to e.g. seek arrival transients occurring at the sample frequency and resulting in off-track data writing operations.
It is also understood that in order to avoid spectral aliasing in a discrete servo control system, it is necessary to apply filtering at a frequency not in excess of one half of the servo sample and control frequency. The half-frequency limit in the sampling control system is known as the Nyquist frequency. In a given actuator design, while the mechanical resonant frequency may lie above the Nyquist frequency, it will appear as an alias in the control spectrum below the Nyquist limit. When a conventional notch filter is applied at the lower alias frequency, the notch filter desensitizes the control system to any tracking errors otherwise lying in the notched-out frequency spectrum. In addition, a low notch frequency may lie dangerously close to the servo loop crossover frequency, and result in degradation of control loop phase margin. What has heretofore been needed has been a notch filter which effectively operates to null out a resonance or disturbance at a frequency above the Nyquist frequency limit of the servo signal sample rate.